…part/form-data

Previously, allowed_signature_methods was being serialized either as a plain string or using indexed keys (e.g., [0], [1]), which resulted in incorrect array parsing by the Mifiel API backend.

This commit updates the serialization to use the `[]` syntax:
  signatories[0][allowed_signature_methods][]=FEA
  signatories[0][allowed_signature_methods][]=FESCV

This is a widely supported format for arrays in multipart/form-data and ensures compatibility with the backend’s expected structure.

Also removes the use of json.dumps, which was turning arrays into escaped strings instead of sending them as proper arrays.

Replaced internal data structure with a list of tuples to correctly handle
multiple allowed_signature_methods for each signer in multipart/form-data.

This change ensures repeated keys are preserved, allowing the backend
to receive arrays like:

  signatories[0][allowed_signature_methods][]=FEA
  signatories[0][allowed_signature_methods][]=FESCV

instead of silently overwriting values when using a dict.

Document.create mistakenly called dict.append when serializing
signatories. Store scalar signatory fields on the dict, collect
allowed_signature_methods as repeated bracket keys, then convert
to a list of tuples for requests when needed. Apply callback_url and
original_hash (dhash) before conversion so dict operations stay valid.

Co-authored-by: Genaro Madrid <genaromadrid@users.noreply.github.com>